Oracle:Joining tables

Joining Tables Introduction

Sometimes information needs to be retrieved from more than one table. The relationships between rows in one table and rows in another are established by the values in certain corresponding columns (foreign key).

For example:

EMP					DEPT
EMPNO	ENAME	JOB	.....	DEPTNO	DEPTNO	DNAME	......
7499	ALLEN	SALES	.....	30	30	SALES	......
				join attributes

The table must have matching values in the join attributes to enable a join to take place. If there are no matching values, the tables will not join! For example, if 30 was missing from the DEPT table, then the employees from department 30 would not appear in any output that joins tables DEPT and EMP together. (A consequence of violating referential integrity!)

Basic SQL Structure For Joining Tables

The format when joining tables is:

SELECT some columns FROM two or more tables WHERE table1.col1 = table2.col2 [ AND table3.col3 = table4.col4 [AND ....]]

Note, two, or more tables can be joined in a SQL statement, but each join condition specifies the link between two tables only. If, for example, three tables appear in the FROM clause, there should normally be two join conditions.

Join Operators

Operator	Description
=	Equal
<>	Not equal
>	Greater than	-	>=	Greater than, or equal to	-	<	Less than	-	<=	Less than, or equal to	-	BETWEEN lower-value AND higher-value	A value between lower and higher
LIKE	Pattern matching

Example Join Queries

To find ALLEN's location:

SELECT  ENAME,  LOC  
FROM  EMP  E,  DEPT  D 
WHERE  ENAME  =  'ALLEN' AND E.DEPTNO = D.DEPTNO;